1. Field of the Invention
This invention relates generally to sirens for emergency vehicles. More particularly, this invention relates to electronic sirens that are used in vehicles to form a wide variety of audio warnings.
2. Description of the Related Art
Emergency vehicles such as fire trucks conventionally use a mechanical siren which sweeps up and down the frequency range (from low pitch, low frequency sound output to high pitch high frequency sound output) to generate a traditional warning which is readily perceptible and recognizable. The siren may also be accompanied by bells and horns to supplement the warning. The pitch of the sound generated by a mechanical siren increases (toward a maximum high frequency pitch) with the rotational speed of the internal rotor. Typically, a switch applies power to the siren drive motor that spins the siren rotor at an increasing speed until a maximum speed is reached. A mechanical siren may take as long as 20 to 30 seconds to achieve maximum speed and thus maximum pitch. When power is removed from the siren drive motor, the siren rotor slows down over time. The pitch of the siren decreases as the rotor slows down. The mechanical siren may also include a brake for rapidly stopping the rotor. Such brakes function similarly to a disc brake in a car and are subject to the same wear and maintenance problems as the rest of the siren assembly. The siren operator in the emergency vehicle controls the up-down sweep of the siren by intermittently closing and opening the switch and/or applying the brake.
While the traditional mechanical siren has functioned well over the years, it is subject to well-known limitations. For example, the mechanical siren is prone to high maintenance because the motor, drive train, brake and bearings wear over time. Mechanical siren sound patterns vary from unit to unit and may vary over time. Additionally, the current draw from a mechanical siren can exceed 100 amps, particularly at startup.
There have recently been attempts to replace the traditional mechanical siren with an electronic siren. Such electronic sirens, which may essentially be a digital recording coupled to an amplifier, require significant power and accordingly place severe demands on the vehicle electrical system. For example, some electronic sirens draw over 30 amps. In addition, the gradually increasing and decreasing pitch pattern produced by a user-actuated mechanical siren is difficult to capture. A digital recording limits the user to playing all, part or repeating parts of the recorded sound pattern. Also, such a recording results in a relatively large digital file, requiring a correspondingly large and thus costly memory capability for storage.
Briefly stated, the invention is a new and improved electronic siren that emulates a mechanical siren by employing a program executed in a micro controller to reproduce the sound patterns produced by a mechanical siren. The program uses timers and look up tables to assemble a digital output having variable frequency, amplitude and harmonic characteristics. A digital to analog (D/A) converter is used to convert the digital output into an analog waveform.
The waveform is then supplied to a Class D amplifier cooperatively linked with a switching power supply and a speaker to provide a reliable solid state system that very closely mimics the volume, harmonic content and sound pattern of a mechanical siren. The efficiency of the switching power supply/class D amplifier combination allows the electronic siren to generate 126 dB of sound pressure at a distance of 10 feet from the reverse folded horn speaker at a current draw of only 10 amps. The micro controller contains programs for generating a number of other selected sounds, including an air horn, a bell, a yelp and a high frequency tone. The electronic siren is configured to selectively receive and amplify the vehicle radio audio signal and may also be equipped with a microphone for use as a public address system.
An object of the invention is to provide a new and improved electronic siren that closely emulates substantially all the sound characteristics of a mechanical siren.
Another object of the invention is to provide a new and improved electronic siren that can generate the necessary sound pressure while demanding relatively little current from the vehicle electrical system.
A further object of the invention is to provide a new and improved electronic siren that has an efficient solid state construction and is adaptable to provide a wide variety of warning sounds.
Other objects and advantages of the invention will become apparent from the specification and the drawings.